Controller for an automatic motor vehicle transmission

ABSTRACT

A control device for an automatic motor vehicle transmission reduces engine torque during a shifting operation, in particular a shifting operation in a traction mode, in order to increase smoothness of shifting. The engine torque reduction is begun at the earliest upon reaching a free-wheel point of the transmission and is ended shortly before a synchronizing point of the transmission is reached. The free-wheel point and the synchronizing point of the transmission are ascertained from rpm differences in the gearshift elements involved in the particular switching operation. Differential rpm thresholds used in the process and the intensity of the torque reduction are ascertained with fuzzy systems.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a control device for an automatic motor vehicletransmission, by which engine torque is reduced in order to increasesmoothness of shifting during a shifting operation, in particular ashifting operation in a traction mode.

Such a controller is used for shifting an automatic motor vehicletransmission. During shifting the engine torque is reduced to enablecomfortable or in other words nonjerking shifting and to keep the powerloss in the friction elements of the transmission as slight as possible.That kind of controller is appropriate for upshifting and downshifting.In downshifting in particular, an increase in comfort is sought, becausein that case there are no power losses, due to the interruption oftraction.

A known transmission controller sends an engagement signal to the enginecontroller when an automatic transmission is shifted. The enginecontroller thereupon changes the engine torque and thus enablesnonjerking, low-wear shifting.

The transmission controller ascertains the proportion by which theengine torque should be varied and informs the engine controller of thatproportion as control information (European Patent 0 518 855 B1,corresponding to U.S. Pat. No. 5,307,270).

In a known method for electronic control of an automatic vehicletransmission with electrohydraulically actuatable friction elements forshifting over among the various gear ratio stages, an actual variablethat characterizes the shifting event is compared with a memorizeddesired variable, and if there is a deviation a correction value isascertained, which adaptively varies the hydraulic pressure for thefriction elements of the transmission (European Patent 0 176 750 B1).Among other factors, the gradient of the transmission input rpm duringshifting of the friction elements is used as the variable thatcharacterizes the shifting event. In order to detect the free-wheelpoint, the chronological derivation of the transmission input rpm mayalso be monitored. In order to detect the synchronization point, thesynchronizing rpm can be determined from the transmission input rpm atthe free-wheel point and from the gear step, with the prerequisite beinga constant output rpm.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a controller foran automatic motor vehicle transmission, which overcomes thehereinafore-mentioned disadvantages of the heretofore-known devices ofthis general type and which ascertains free-wheel and synchronizationpoints of the transmission from rpms in the transmission that are simpleto determine.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a control device for an automatic motorvehicle transmission, for reducing engine torque to increase smoothnessof shifting during a shifting operation, in particular a shiftingoperation in a traction mode, comprising beginning the engine torquereduction at the earliest upon attainment of a free-wheel point of thetransmission and ending the engine torque reduction shortly before asynchronizing point of the transmission is reached; and ascertaining thefree-wheel point and the synchronizing point of the transmission on thebasis of rpm differences between gearshift elements involved in a givenshifting operation.

In accordance with another feature of the invention, the rpm differencesof transmission gearshift elements, each including two gearshift elementhalves, are ascertained from a transmission input rpm, an output rpm andrpm factors resulting from gear tooth ratios in the transmission.

In accordance with a further feature of the invention, the engine torqueis reduced as soon as the rpm difference of a given engaging gearshiftelement is not equal to zero.

In accordance with an added feature of the invention, the engine torqueis returned to its outset value as soon as the rpm difference of thegiven gearshift element to be shifted exceeds a predetermined thresholdrpm.

In accordance with a concomitant feature of the invention, there isprovided a fuzzy system defining the threshold rpm, the fuzzy systemevaluating a predetermined intensity of the engine torque reduction, astandardized engine torque at the beginning of shifting, and astandardized differential rpm at the engaging gearshift element, asinput variables.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a controller for an automatic motor vehicle transmission, it isnevertheless not intended to be limited to the details shown, sincevarious modifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic and block diagram showing a layout of an automatictransmission according to the invention;

FIG. 2 is a schematic diagram showing components of the transmission ofFIG. 1 that are essential to computer ascertainment of transmissiondata;

FIG. 3 is a graph showing a course over time of an output moment of atransmission in the case of traction shifting;

FIG. 4 is a graph showing courses of a differential rpm of one gearshiftelement to be shifted and one gearshift element to be disengaged in thetransmission of FIG. 2;

FIG. 5 is a block diagram of a fuzzy system used in the controller ofthe transmission of FIG. 1 or 2;

FIG. 6 is a flow chart of a program for detecting an onset and an end ofa moment reduction;

FIG. 7 is a block diagram of a fuzzy system for parametrizing the momentreduction;

FIGS. 8-10 are graphs of membership functions of input and outputvariables of the fuzzy system of FIG. 7;

FIGS. 11 and 12 are graphs showing a course of the output torque and theengine torque in traction downshifting without moment reduction; and

FIGS. 13 and 14 are graphs showing a course of the output torque and theengine torque in traction downshifting with moment reduction.

FIG. 15 is a diagram showing a control of a shifting transition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the figures of the drawings in detail and first,particularly, to FIG. 1 thereof, there is seen a drive 1 of a motorvehicle that includes a transmission 2 which has a planet wheel set 3that is constructed, for instance, as a Ravigneaux, Simpson or Wilsontype of construction, for rpm and torque conversion. A hydrodynamictorque converter 4, which is also known as a Fottinger converter, isused as a startup element that can be bypassed with a non-illustratedmechanical clutch to improve efficiency. The transmission is controlledby an electrohydraulic controller 6, which receives commands and datafrom the driver and the motor vehicle and exchanges data with the engineas is indicated in the drawing. The controller 6 includes an electroniccontrol unit and a hydraulic actuation part, controlled by theelectronic control unit, which supplies pressure fluid to actuators forvarious friction elements in the transmission, that is clutches, brakesand free wheels.

The automatic transmission 2 in the example described herein isconstructed as a converter-type, four-speed transmission. Its basiclayout can be seen from FIG. 2, in which a planetary gear on theRavigneaux principle and a plurality of clutches and brakes are shown torealize the gear shifting operations, but not to realize the generallyknown hydrodynamic converters 4. The components of the transmission areas follows: an input or drive shaft An, an output or power takeoff shaftAb, a first clutch K1, a second clutch K2, a first brake B1, a secondbrake B2 and a free wheel F. The gear wheels of the transmission areidentified by their numbers of teeth listed as follows: z1=26 for asmall sun wheel, z2=34 for a large sun wheel, z3=22 for a short planetwheel, z4=20 for a long planet wheel and z5=74 for a hollow wheel.

A fast gear (overdrive) is realized by connecting a transmission inputwith a support member of the first gear (strut) through an additionalclutch. The support member both in fourth gear and second gear is thelarge sun wheel. Through the use of this kind of construction, it ispossible to dispense with one additional groove to achieve the overdrivegear.

The two primary tasks of an electronic transmission controller arecontrolling the shifting point and controlling the shift transition.Shifting point control is carried out in a known manner throughperformance graphs (where a memorized throttle valve position is plottedover the output rpm). The choice of performance graphs stored in thecontrol unit (such as "sporty", "economy" and "winter") is carried outmanually or adaptively by the evaluation of variables measured while thevehicle is in operation.

By far the more-complex part of the electronic transmission controlleris the control of the shifting transition. With its aid, the "softest"possible gear change is sought. In order to accomplish this, not onlymust the pressure in the friction elements involved in the shifting bevaried, but many other activities, shown in the table below, arenecessary. A shifting transition that meets modern demands for anautomatic transmission can be attained only by using so-called smartcontrol electronics. In terms of circuitry (hardware) this requires alarge scale of component integration and high performance on the part ofthe microcontrollers being used, since a large number of data must beprocessed in a very short time.

The two essential aspects of control are the control of engine torque(also called engine moment control) and pressure control, as shown inFIG. 15. In engine moment control, intervention into the enginecontroller is made during shifting. To that end, data are exchangedbetween the transmission controller and the engine controller. Theshifting and release of the gearshift elements in the transmission arecontrolled with the aid of the pressure control. To that end, a pressuremodulation is performed with proportional or fast-switching valves. Bothaspects are closely related and affect one another. The pressure controlin shifting operations is described in co-pending U.S. patentapplication Ser. No. 08/758,385, filed on the same day as the instantapplication.

In the context of the present invention, a fuzzy-logic-based shiftingtransition controller was developed and tested with a "closed-loop"simulation. The main emphasis was traction shifting. The transmission 2is constructed as a four-speed Ravigneaux set with a Fottingerconverter.

Essentially, the following steps were carried out:

preparation of a simulated model for a four-speed Ravigneauxtransmission within a simple drive train;

construction of a model for the hydraulic controller;

construction of a fuzzy-logic-based engine moment controller;

construction of a fuzzy-logic-based pressure controller;

testing of the shifting transition control in the simulation.

The use of fuzzy systems for controlling the automatic transmission 2was chosen because of the complexity of the stated object and the manyvariables to be processed. Moreover, such a system can be preparedrelatively quickly with available development systems (tools).

The control of the engine moment when shifting in the traction mode willbe described below. It is also possible in principle to vary the courseof shifting for overrunning shifting through the use of an engineintervention. In order to vary the engine moment, the following optionsare available: adjusting ignition angle, fadeout of injections,electronic adjustment of the throttle valve and electronic adjustment ofthe idling charge. Since the electronic adjustment of the throttle valveand idling charge is relatively complicated to realize and involvesinertia, and because it is unfavorable to vary the exhaust gas behaviorby fading out injections, the ignition angle adjustment is used in thiscase.

The control of the engine moment in traction shifting operations isdescribed below. Traction shifting operations are the most criticalshifting operations in terms of the power loss occurring in thegearshift elements and the resultant thermal strain on the frictionlinings. If the engine moment is not reduced during shifting,destruction of the friction linings occurs very easily. With an enginemoment controller according to the invention, the excessivemultiplication in output torque M_(out), shown in FIG. 3, in the inertiaphase of shifting a conventional transmission can be reduced, so thatthe transmission output moment is smoothed and there is a markedimprovement in passenger comfort.

Equation 1 below shows that it is possible to reduce the lost work thatoccurs in the engaging gearshift element (free-wheel shifting), byreducing the engine moment. ##EQU1##

The symbols in the equation are as follows:

    ______________________________________                                        Q.sub.v   lost work                                                           t.sub.s   shift time                                                          i.sub.new gear ratio of the new gear                                          i.sub.old gear ratio of the old gear                                          J.sub.in  moment of inertia on input side of transmission                     M.sub.eng engine moment                                                       n.sub.out transmission output rpm                                             ω.sub.out                                                                         output angle speed                                                  ______________________________________                                    

From this equation it can be seen that the lost work converted in theclutch is composed of a kinetic portion (braking of the engine masses)and a portion arising from the combustion in the engine. The possibilityexists, by reducing the engine moment, of decreasing the lost work inthe gearshift elements that occurs during shifting. It also becomesclear that upon a reduction in engine moment, the slip time of thegearshift element can be increased, without increasing the lost work.This increase in available slip time improves the smoothness ofshifting.

In evaluating the possibilities created by the engine momentintervention, the product of the engine moment and the shifting time isdecisive. The shifting time is considered to be a function of the clutchmoment, since that can be varied from outside. If the engine moment isnot varied, then to achieve a slip time that is not critical to themagnitude of the lost work, a high clutch moment must be brought tobear. If the clutch moment remains constant and the engine moment isreduced, then the slip time becomes shorter and thus there is less lostwork. Conversely, if the slip time is to remain constant despite areduction in the engine moment, then the clutch moment can be reducedaccordingly. What is important is to optimize the length of the sliptime with respect to shifting smoothness and strain on the frictionelements.

The sequence control of the engine intervention in traction shiftingoperations will now be described. The main problem in sequence controlis the precise adaptation of the timing of the engine intervention. Itis crucial that the engine moment not occur before the gearshift elementthat carries the old gear is released completely (free-wheel point). Upto that time, the gear ratio of the old gear is retained unchanged, anda reduction in engine torque would lead to an increased sag in torque inthe torque phase. Accordingly, it is necessary that the free-wheel pointbe detected exactly. When the free-wheel point is reached, the enginetorque can immediately be reduced to a predetermined value.

The instant for ending the engine intervention must be chosen in such away that the restoration of the engine torque to its initial valueoccurs as shortly as possible before the synchronizing point (end ofshifting) and the regulation upward until the end of shifting ischronologically feasible. Proposals thus far take their point ofdeparture, as noted, for instance in monitoring the chronologicalderivation of the transmission input rpm to detect the free-wheel point.In order to detect the synchronizing point, the synchronizing rpm can bedetermined from the transmission input rpm at the free-wheel point, andthe gear step, on the condition that the output rpm is constant. Shortlybefore the synchronizing rpm is reached, the torque is scaled back toits original value.

In the context of the present invention, the free-wheel point and thesynchronizing point are determined from the differential rpm valuesinvolved in the shifting. These rpm values of the gearshift elementhalves may be determined from the transmission input rpm, the output rpmand the rpm factors. If the transmission input rpm is not available,then calculation can also be carried out with rpm of an internal shaftin the transmission, since the input rpm ω_(in), output rpm ω_(out) andcoupling shaft rpm ω_(i) are always linked through the followingequation 2:

    ω.sub.i =a.sub.i -ω.sub.in +b.sub.i +ω.sub.out. (2)

The rpm factors a_(i) and b_(i) are ratios of numbers of teeth andcharacterize the angular speeds of the corresponding coupling shafts asa function of the angular speeds of the drive and output. The drive andoutput are each considered to be fixed for calculation purposes. Therelationship that applies is

    a.sub.1 +b.sub.1 =1.

The rpm factors of the gearshift element halves must be calculated forthe corresponding transmissions, and must be available, given anassociation with the type of shifting (1-2, 2-3, . . . ) in a memory,which is represented by the following table.

    ______________________________________                                                   rpm factors of the rpm factors of the                              Shifting type                                                                            disengaging clutch shifting clutch                                 ______________________________________                                        1-2        a.sub.l11,b.sub.l11                                                                   a.sub.l21,b.sub.l21                                                                      a.sub.s11,b.sub.s11                                                                 a.sub.s21,b.sub.s21                       2-3        a.sub.l12,b.sub.l12                                                                   a.sub.l22,b.sub.l22                                                                      a.sub.s12,b.sub.s12                                                                 a.sub.s22,b.sub.s22                       3-4        a.sub.l13,b.sub.l13                                                                   a.sub.l23,b.sub.l23                                                                      a.sub.s13,b.sub.s13                                                                 a.sub.s23,b.sub.s23                       ______________________________________                                    

In the table:

a is the rpm factor referred to the drive

b is the rpm factor referred to the output

l is the disengaging gearshift element

s is the engaging gearshift element

The 1st digit stands for the gearshift element half

The 2nd digit stands for the shifting type

The rpms of the gearshift element halves involved in the shifting canthus be determined. The differential rpm values of the halves of thegearshift elements can then be calculated in turn from these individualrpm values, using the following equations 3 and 4:

disengaging clutch: (for example: shifting 1-2)

    n.sub.111 =a.sub.111 ·n.sub.in +b.sub.111 ·n.sub.out

    n.sub.121 =a.sub.121 ·n.sub.in +b.sub.121 ·n.sub.out

    abs(ndiff.sub.11)=abs(n.sub.111 -n.sub.121)                (3)

where:

n₁₁₁ is the rpm of the first half of the disengaging gearshift elementfor the shifting transition 1-2

n₁₂₁ is the rpm of the second half of the disengaging gearshift elementfor the shifting transition 1-2

ndiff₁₁ is the differential rpm of the disengaging gearshift element forthe shifting transition 1-2

engaging clutch: (for example: shifting 1-2)

    n.sub.s11 =a.sub.s11 ·n.sub.in +b.sub.s11 ·n.sub.out

    n.sub.s21 =a.sub.s21 ·n.sub.in +b.sub.s21 ·n.sub.out

    abs(ndiff.sub.s1)=abs(n.sub.s11 -n.sub.s21)                (4)

where:

n_(s11) is the rpm of the first half of the engaging gearshift elementfor the shifting transition 1-2

n_(s21) is the rpm of the second half of the engaging gearshift elementfor the shifting transition 1-2

ndiff_(s1) is the differential rpm of the engaging gearshift element forthe shifting transition 1-2

Thus for the current shifting operation the differential rpm values atthe two gearshift element values are available. The free-wheel point(differential rpm of the disengaging gearshift element becomes greaterthan zero) and the synchronizing point (differential rpm of the engaginggearshift element becomes zero) can then be determined exactly with theabsolute values of these differential rpm values. The courses of thedifferential rpm values of the shifting and the disengaging gearshiftelement can be seen from FIG. 4.

As already suggested earlier herein, upon detection of the "free-wheelpoint", the engine torque can be reduced immediately. This reductionmust be terminated shortly before the synchronizing point, so thatenough time will still remain for the scaling back. In order toaccomplish this, a threshold rpm difference is ascertained, so that ifthis threshold fails to be attained by the rpm difference of theshifting clutch, the engine intervention is terminated. The reducingengine torque is then returned to its initial value again.

The defining of the threshold rpm is carried out through the use of afuzzy algorithm, which proceeds in a fuzzy system 11 (FIG. 5). Thisalgorithm is called up at the beginning of the shifting operation andincludes the following input variables 12-14, which are processed in arule base 15:

12=intensity of torque reduction (factor 0 . . . 1);

13=standardized engine torque at onset of shifting: ##EQU2##14=standardized differential rpm at the gearshift element to be shifted:##EQU3##

During the traction shifting operation, the engine torque does not varysubstantially, because of the precautions taken. It is thereforepossible to estimate the step in torque necessary for the resetting,from the engine torque at the onset of shifting and the magnitude oftorque reduction. There must be enough time available to complete thistorque step, for instance through the use of an ignition angleadjustment.

The value of the differential rpm at the onset of shifting, givenrelatively constant shifting times, allows a conclusion to be drawn asto the magnitude of the negative rise in the course of the differentialrpm. If the rise is strongly negative, the differential rpm thresholdmust be raised, because otherwise the time remaining until the end ofshifting is too short for the torque resetting.

An output variable 16 of the fuzzy algorithm is a standardized thresholdrpm nsw_(norm), with standardization being performed to a differentialrpm of 130*1/min.

The structure of the fuzzy algorithm can be seen from FIG. 5. In it thefollowing variables are used:

kredf₋₋ zh: intensity of torque reduction for traction upshifting,

m0₋₋ norm: standardized engine torque,

ndsk₋₋ norm: standardized differential rpm of the engaging gearshiftelement,

nsw₋₋ zh₋₋ norm: standardized rpm threshold.

In FIG. 6, the entire algorithm for detecting the beginning and end oftorque reduction for traction upshifting is shown in the form of a flowchart.

The flowchart is self-explanatory. It can be seen that in the converterrange, no variation of torque is performed, so that the shiftingproceeds at very low torques. The detection of the converter range iscarried out by monitoring the course of the converter amplification μ.

The intensity of torque reduction in traction shifting operations willnow be explained. As already indicated earlier above, the goal of torquereduction in traction upshifting is to improve shifting smoothness andreduce the lost work occurring during the shifting transition. Theimprovement in shifting smoothness is attained because the requisiteclutch moment for shifting can be reduced by reducing the torque. Thisaverts a strong multiplication of the output torque in the inertiaphase. The possibility of reducing the lost work has been shown withrespect to equation 5.14.

The system explained herein for varying the torque parametrizes theintensity of torque reduction at the onset of traction upshifting. Thisrequires that through the use of conclusive variables at the onset ofshifting, a conclusion can be drawn about the lost work to be expected,and defining the intensity of torque reduction accordingly. This iscarried out with the aid of two input variables: gear step andtransmission input rpm. The choice of these two variables is based onthe fact that the lost work is determined essentially by the shiftingtime, which in turn is determined by the gear step and the transmissioninput rpm (equation 5.9). An increase in the two variables brings aboutan increase in lost work. If the shifting time is kept fairly constantthrough the use of the pressure control, then upon a torque reductionthe requisite clutch moment can be reduced, which leads to animprovement in shifting smoothness.

In order to parametrize the intensity of torque variation, a fuzzysystem 18 (FIG. 7) is used, which generates the intensity of the torquereduction as an output value 22 (FIG. 7) from the input variables ofgear step 19 and transmission input rpm 20 in a rule base 21. Thisoutput value serves as a reducing factor for the further processingwithin the transmission controller. The term parametrizing is understoodin this case to mean the precise ascertainment of a variable, usingknown or derivable transmission parameters.

The use of the fuzzy system is especially advantageous because there ispractically no resultant need for a sharp distinction between the"transmission input rpm" input variable and the "reducing factor" outputvariable.

The variables in FIG. 7 have the following meanings:

igs₋₋ fuzzy: gear step,

n1₋₋ fuzzy: standardized transmission input rpm,

kred₋₋ zh: factor of torque reduction for traction upshifting 0 . . .1!,

nsw-rules: rules of rule base 21.

The transmission input rpm has thus been standardized to its maximumvalue.

The fuzzy sets of input and output variables of the fuzzy system 18 canbe seen from FIGS. 8, 9 and 10. They show the membership functions ofthe three fuzzy variables, that is the degrees of membership of thevariables igs, n1₋₋ fuzzy, and kred₋₋ zh.

The rule base 21 includes the following fuzzy rules:

RULEBASE mehs₋₋ rules

RULE Rule1

IF(igs₋₋ fuzzy IS LOW)AND(n1₋₋ fuzzy IS VL)THEN kred₋₋ zh=HIGH END

RULE Rule2

IF(igs₋₋ fuzzy IS LOW)AND(n1₋₋ fuzzy IS LOW)THEN kred₋₋ zh=HIGH END

RULE Rule3

IF(igs₋₋ fuzzy IS LOW)AND(n1₋₋ fuzzy IS MED)THEN kred₋₋ zh=HIGH END

RULE Rule4

IF (igs₋₋ fuzzy IS LOW)AND(n1₋₋ fuzzy IS HIGH)THEN kred₋₋ zh=MED END

RULE Rule5

IF(igs₋₋ fuzzy IS LOW)AND(n1₋₋ fuzzy IS VH)THEN kred₋₋ zh=MED END

RULE Rule6

IF(igs₋₋ fuzzy IS MED)AND(n1₋₋ fuzzy IS VL)THEN kred₋₋ zh HIGH END

RULE Rule7

IF(igs₋₋ fuzzy IS MED)AND(n1₋₋ fuzzy IS LOW)THEN kred₋₋ zh=HIGH END

RULE Rule8

IF(igs₋₋ fuzzy IS MED)AND(n1₋₋ fuzzy IS MED)THEN kred₋₋ zh=MED END

RULE Rule9

IF(igs₋₋ fuzzy IS MED)AND(n1₋₋ fuzzy IS HIGH)THEN kred₋₋ zh=MED END

RULE Rule10

IF(igs₋₋ fuzzy IS MED)AND(n1₋₋ fuzzy IS VH)THEN kred₋₋ zh=LOW END

RULE Rule11

IF(igs₋₋ fuzzy IS HIGH)AND(n1₋₋ fuzzy IS VL)THEN kred₋₋ zh MED END

RULE Rule12

IF(igs₋₋ fuzzy IS HIGH)AND(n1₋₋ fuzzy IS LOW)THEN kred₋₋ zh=MED END

RULE Rule13

IF(igs₋₋ fuzzy IS HIGH)AND(n1₋₋ fuzzy IS MED)THEN kred₋₋ zh=MED END

RULE Rule14

IF(igs₋₋ fuzzy IS HIGH)AND(n1₋₋ fuzzy IS HIGH)THEN kred₋₋ zh LOW END

RULE Rule15

IF(igs₋₋ fuzzy IS HIGH)AND(n1₋₋ fuzzy IS VH)THEN kred₋₋ zh=LOW END

END

END

The results of torque reduction in traction downshifting operations willnow be explained in conjunction with FIGS. 11-14. FIGS. 11 and 12 showthe course over time of the output torque M_(out) or engine torqueM_(eng) in traction downshifting without torque reduction. The sharpstep in output torque (marked with an arrow) when the free-wheel catchescan be clearly seen. The torque intervention seeks to reduce this stepin torque that is associated with an acceleration jerk and a rotation ofthe output shaft.

FIGS. 13 and 14 show the course over time of the output torque M_(out)or the engine torque M_(eng) in a traction downshifting operation inwhich a torque reduction according to the invention has been performed.The smoothing of the torque step at the output shaft, effected by thetorque reduction, at the end of the shifting operation, wherein the endis definitive for gentle shifting, is clearly apparent (upper arrow inFIG. 13). Through the use of the invention, the smoothness of shiftingof the vehicle drive 1 provided with an automatic transmission 2 is thusmarkedly improved, and the lost work occurring during shifting isreduced.

We claim:
 1. In a control device for an automatic motor vehicletransmission, for reducing engine torque to increase smoothness ofshifting during a shifting operation, the improvement comprising:meansfor beginning the engine torque reduction not before attainment of afree-wheel point of the transmission and for ending the engine torquereduction shortly before a synchronizing point of the transmission isreached; and means for ascertaining the free-wheel point and thesynchronizing point of the transmission on the basis of rpm differencesbetween gearshift elements involved in a given shifting operation. 2.The control device according to claim 1, wherein the shifting operationoccurs in a traction mode.
 3. The control device according to claim 1,including means for ascertaining the rpm differences of transmissiongearshift elements, each including two gearshift element halves, from atransmission input rpm, an output rpm and rpm factors resulting fromgear tooth ratios in the transmission.
 4. The control device accordingto claim 1, including means for reducing the engine torque as soon asthe rpm difference of a given engaging gearshift element is not equal tozero.
 5. The control device according to claim 4, including means forreturning the engine torque to its outset value as soon as the rpmdifference of the given gearshift element to be shifted exceeds apredetermined threshold rpm.
 6. The control device according to claim 5,including a fuzzy system defining the threshold rpm, said fuzzy systemevaluating a predetermined intensity of the engine torque reduction, astandardized engine torque at the beginning of shifting, and astandardized differential rpm at the engaging gearshift element, asinput variables.